Our long-term goals are 1) to understand the mechanism that generates circadian rhythmicity in the retina and 2) to elucidate the role played by rhythmic synthesis of melatonin in retinal function. We will begin by defining optimal culture conditions for studying circadian rhythms of melatonin synthesis in in vitro preparations of isolated neural retinas from rats and mice. We will then measure the activity of the enzymes involved in melatonin synthesis and the breakdown products produced by its catabolism in order to define the proximate source of the circadian rhythmicity. We will determine whether dopamine is also produced rhythmically by cultured retinas. Using mice and rats with genetic defects that affect specific cell types in the retina we will ask which cell types are essential for synthesis and degradation of melatonin (and dopamine) and which are essential for circadian oscillation. We will define the response of the retinal circadian oscillators to light pulses and explore the light input pathway using pharmacological agents that block or mimic the resetting effects of light. Finally we will ask whether melatonin is causally involved in synchronized rhythmic outer segment disc shedding by comparing disc shedding in C3H mice, whose retinas synthesize melatonin rhythmically, with disc shedding in C57 mice whose retinas, like their pineal glands, make no melatonin at all. What we learn about rhythmicity in mammalian retinas in culture is likely to be applicable to pathologies of retinal function in humans, particularly since so many aspects of retinal physiology have been shown to be rhythmic.